Abstract: In response to the challenges of sand production and high water cut during the exploitation of oil reservoirs in unconsolidated sandstones, a novel sand–water dual-control functional polymer, PDSM, was synthesized using acrylamide (AM), methacryloxyethyltrimethyl ammonium chloride (DMC), and styrene monomer (SM) as raw materials. The chemical structure and thermal stability of PDSM were verified by 1H-NMR, FT-IR, and TGA analyses. To evaluate its performance, functional polymers PDM and PSM, containing only DMC or SM, respectively, were used as control groups. The study systematically investigated the static adsorption, sand production, sand leakage time, standard water–oil resistance ratio, and water cut reduction performance of PDSM. The results demonstrated that, due to the synergistic effect of functional monomers DMC and SM, PDSM exhibited superior dual-control over sand and water compared to PDM and PSM. PDSM enhanced wettability properties reduce the contact angle of the water phase on oil-wet rock surfaces to 64.0°, facilitating better adsorption of polymer molecules on the rock surface and achieving a static adsorption capacity of 14.6 mg/g. PDSM effectively bridges/bundles sand grains through SM and DMC, increasing resistance to fluid erosion. At a flow rate of 100 mL/min, sand production was only 0.026 g/L, surpassing the “Q/SH 1020 2377-2020” standard for sand inhibitors, which defines "excellent" performance as having a sand production rate of ≤ 0.05 g/L. PDSM forms an adsorption layer (polymer concentrated layer) on the rock surface, expanding when in contact with water and shrinking when in contact with oil, thereby significantly reducing the permeability of the water layer without affecting the permeability of the oil layer. The standard water–oil resistance ratio was measured at 5.41, and the water cut of produced fluid was reduced by 18.6%. These findings provide new theoretical insights and technical guidance for developing dual-function sand–water control agents.